German patent document DE 20 2005 016 686 U1 discusses an electrical power supply to an electronic brake system of a trailer, in which an electronic brake system which is arranged in the trailer can be supplied with power via two different cables. One of the cables serves to supply power to the trailer brake lights and has a branch which leads to the electronic brake controller. The second cable has a separate cable for the electronic brake controller. If said second cable is not connected or correct power supply cannot be ensured for other reasons, electrical power is applied to the electronic control device via the first cable at least during a braking operation.
German patent document DE 10 2004 008 935 A1 discusses an apparatus for redundant voltage supply for a control device in an on-board vehicle electrical system having two lines, of which the first is designed as a main connection for transferring all the power, while the second line is configured as an emergency supply line and is designed such that only relatively low power consumption is possible. The two lines are decoupled by diodes.
In accordance with the Agreement Concerning The Adoption Of Uniform Technical Prescriptions For Wheeled Vehicles, §5.1.3.6 of ECE R13 allows the alternative supply of electrical power to an electronic brake system (EBS) in the trailer vehicle via a plug-type connection according to ISO standard 1185/ISO standard 12098, with this type of supply having to be restricted to the case of “failure of the electrical power supply via the plug-type connection according to ISO 7638”.
In order to clarify the addressed problem, FIG. 2 shows an electronic control device 1 with five connection pins P1-P5. The connection pin P1 is connected to battery voltage and is wired in accordance with ISO 7638 (PIN 1 there). PIN 2 is connected to the brake light according to ISO 1185 (PIN 4 there) and is likewise supplied with supply voltage when a brake light switch (not illustrated) is operated. These two supply voltages are decoupled by diodes D1 and D2.
PIN P3 is wired in accordance with ISO 7638 (PIN 2 there). PIN P4 is connected to ground, specifically in accordance with ISO 7638 (PIN 4 there), while PIN P5 is wired in accordance with ISO 1185 (PIN 1 there) and is likewise connected to ground. PIN P3 is wired in accordance with ISO 7638 (PIN 2 there) and is accordingly connected to supply voltage when the ignition (IGN) is switched on. The PINs P1 and P4 and also P2 and P5 therefore each form a power supply for the electronic control device 1, so that redundant power supply is present. The ground connections of the two redundant power supplies by ISO 7638 and ISO 1185 must not be directly connected to one another, that is to say the connection illustrated by a dashed-line circle K1 is impermissible since otherwise an electrical load, for example a vehicle lighting system L2, reacts to the electronic control device 1 and therefore a brake system and also is effective when there is no fault.
On account of the relatively large cross section of the ground line of the ISO 7638 plug-type connection (PIN P4), a compensation current I2 would flow in the illustrated direction, as a result of which an additional voltage drop across the ground connection of the ISO 7638 plug-type connection (PIN P4) would be caused, this resulting in an increase in the ground potential, that is to say the common potential of PINs P4 and P5. Depending on the magnitude of the actual supply voltage at PIN P1, this could lead to an undervoltage of the electronic control device 1. If the ground line to PIN P5 is interrupted, this being indicated by the interruption point B1, the current I1 can no longer flow away via said ground line, and therefore amplifies this effect since the entire current can then flow away only as current I3 via the ground line of PIN P4.
L1 denotes a brake light which is situated between PINs P2 and P5. L2 indicates a vehicle lighting system, of which the ground connection is connected to PIN P5.
FIG. 3 shows the system which has been used to date to solve the problem described in conjunction with FIG. 2. The two ground lines to PINs P4 and P5 are decoupled by diodes D5 and D6, and therefore the current I2 shown in FIG. 2 can no longer flow.
However, decoupling by diodes D5 and D6 results in the following disadvantages:                the internal ground potential M1 is raised by the voltage drop across diodes D5 and D6;        the maximum continuous current is restricted since, otherwise, an excessively high power loss is produced in diodes D5 and/or D6;        the availability of the electronic control device 1 is reduced by the diode voltage of the diodes D5 and D6;        the current of the electronic control device 1 can also flow away via the brake light ground connection (ground line to PIN P5) and not only, as would be correct, via PIN P5, that is to say the ground line according to ISO 7638.        